Until recently Ethernet switching has been based on simple multiplexing techniques without traffic shaping features, such as FIFO queues and forwarding priorities.
With the IEEE 802.1Qav standard (FQTSS for “Forwarding and Queuing for Time-Sensitive Streams”), which is part of the “Ethernet AVB” standard suite, per class traffic shaping has been introduced to handle rate constrained traffic.
With Ethernet TSN (Time Sensitive Networking), another traffic shaping scheme has been added for the support of scheduled traffics: 802.1Qbv (Enhancements for scheduled traffic). This multiplexing mechanism organises the transmission multiplex of a switch output port in time windows reserved for different traffic classes. This makes possible the accurate definition of time intervals (absolute starting and ending times of the interval) that are reserved at particular positions in time for the transmission of frames which can suffer only a tightly bounded transmission latency.
Standard 802.1Qbv is intended for bridges or switches to support transmission from queues that are scheduled relative to a known timescale. This timescale is shared by all the nodes (switches) belonging to the domain supporting scheduled traffics. Timing and synchronisation information is distributed using a specific protocol such as PTP (Precision Time Protocol, IEEEP1558) or gPTP (generalised PTP, IEEE 802.1AS).
As shown in FIG. 1, a transmission gate is associated with each queue: the state of the gate (C; o) indicates if queued frames can be selected for transmission.
For a given queue, the gate can be in one of two states:
Open (o): queued frames can be transmitted according to the transmission selection mechanism associated with the queue, and
Closed (C): queued frames are not selected for transmission.
The queues are located in a given port with which a gate control list is associated. The gate control list contains an ordered list of gate operations. Each gate operation changes the transmission gate state.
A state machine controls the execution of the gate control list.
A frame of a transmission class queue cannot be transmitted if the associated transmission gate is in the Closed state or if there is insufficient time to transmit the entire frame before the next gate-close event.
IEEE 802.1Qbv provides means for accurately reserving time intervals on the transmission link of a switch transmission port. These time intervals are dedicated for scheduled frames of given traffic classes.